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ADS131E08: ADS131E08 Standard Input range Issues

Dylan Kemp
Prodigy 10 points
Part Number: ADS131E08
Other Parts Discussed in Thread: ADS131M08, ADS127L18

Tool/software:

How do I simply read 0V - 2.4V and get the full range using the ADS131E08?

This is my current schematics:

AVDD = 3.3V

AVSS = 0V

INTERNAL VREF = 2.4V

All my ADC signals range from 0V-2.4V . 

Is this the wrong 24bit ADC chip to use in this scenario?

  • 0
    TI__Mastermind 23275 points

    Hi Dylan,

    welcome to our e2e forum and thanks a lot for choosing a TI ADC.

    The ADS131E08 is unfortunately not a good choice when you want to measure single-ended signals (i.e. negative analog input is tied to GND), when using a unipolar analog power supply (i.e. AVSS = GND = 0V).

    The integrated PGAs need a certain common-mode range to operate as mentioned in the "9.3.4.1 Input Common-Mode Range" section in the datasheet.
    If you are using Gain = 1, that would mean both the positive and negative analog inputs always need to be >=0.3V.
    When you tie the negative input to GND, the device will work, but you will see offset, gain, and INL errors.
    To get around this issue you either have to level shift your inputs or you need to use a bipolar analog supply (i.e. AVSS < GND-0.3).

    The second issue you might experience is that most of our delta-sigma ADCs always measure differentially, even when you configure the inputs for single-ended measurements. The 2^24 codes are mapped across the differential input range from -VREF/Gain to +VREF/Gain. In your case you only leverage the code range from 0V to + VREF/Gain = 2.4V. That means you don't leverage half the code range, which isn't necessarily a problem.
    If you look at section "9.5.1 Data Format" in the datasheet, you will realize that the code for a +2.4V signal would be 0x7FFFFF in your case.

    Regards,
    Joachim Wuerker

  • 0 in reply to Joachim Wuerker
    Prodigy 10 points

    Thanks for you quick reply and explaination.

    Could you recommend a 24bit adc that has simultaneous reading of all 8CH that would suit single ended uni polar design?

  • 0 in reply to Dylan Kemp
    TI__Mastermind 23275 points

    Hi Dylan,

    all our 24-bit, simultaneous-sampling ADCs have the "issue" that you can only leverage half the code range when you perform single-ended measurements. However that is usually not an issue, because the noise will anyway be larger than your LSB size. So you are not losing any performance unless the ADC would be 24-bit noise free. I wouldn't worry about this at all.

    I would have pointed you to the ADS131M08, but unfortunately you could only measure single-ended inputs up to 1.2V with that device.

    You might have to go with our new ADS127L18 in that case. That is an extremely high-performance ADC.

    Regards,
    Joachim Wuerker